SECTION X. 



THE RENAL SECRETION. 



THE blood not only bears to the different tissues the substances 

 required for their nutrition, but also removes from the tissues the 

 different waste products which result from their A r arious metabolic 

 processes. In the lungs, part of these oxidation products, especially of 

 the carbon compounds, are removed, while the results of nitrogenous 

 waste largely pass through the lungs to be carried through the aorta 

 from the left ventricle to the kidneys, whose function is to remove these 

 nitrogenous excrementitious substances, together with the carbon com- 

 pounds which have passed the lungs, with various salts and water. The 

 product of this functional activity of the kidneys is the urine, which is 

 a pure excretion, since all its constituents are waste products which 

 must be removed from the organism. 



1. THE PHYSICAL AND CHEMICAL PROPERTIES OF URINE. Urine is 

 in general a thin, yellowish colored, transparent fluid (the depth of color 

 depending on the concentration) of a salty taste and peculiar aromatic 

 odor, due to the presence of various volatile acids. Its reaction may 

 be faintly acid, neutral, or alkaline : it rotates the plane of polarized 

 light to the left. The reaction in the carnivora and in fasting or 

 suckling herbivora is acid ; in the herbivora and omnivora, when on 

 vegetable diet, it is alkaline. The explanation of the production of an 

 acid renal secretion from the alkaline blood is to be attributed to a 

 specific property of the renal epithelium similar to that possessed by the 

 gastric mucous membrane. The more alkaline the blood, the less acid 

 the urine ; hence the great alkalinity of the blood of herbivora causes 

 the urine to have an alkaline reaction. For although sulphuric acid is 

 formed from the decomposition of vegetable just as it is from animal 

 albumen, vegetable foods contain large amounts of organic salts which 

 break up into alkaline carbonates, and so neutralize the sulphuric acid. 

 These salts are absent from the food of carnivora, hence the acids are 

 less perfectly neutralized. It is, therefore, the form of diet which, by 

 modifying the alkalinity of the blood, determines the reaction of 

 the urine. The specific gravity of urine varies between 1005 and 1050. 

 When exposed to the air, the urea undergoes decomposition and is 

 transformed into ammonium carbonate ; a part of the ammonia then 

 combines with magnesium phosphate, and ammonium-magnesium phos- 



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